The present invention relates to an optical information storage apparatus capable of recording, reproducing or erasing information by utilizing light irradiation. More particularly, the present invention relates to a stack type optical disc apparatus including a plurality of stacked optical disc mediums which is capable of easily raising the density and enlarging the capacity; to a sealed type separation optical head therefore; and to an optical disc medium (information recording medium).
In accordance with the progress of the information-oriented society, a data file apparatus having a large capacity has been desired. Therefore, there has been known a stack type optical disc apparatus serving as a large capacity data file apparatus capable of meeting the above-described desire. The stack type optical disc apparatus is arranged in such a manner that a plurality of optical disc mediums are simultaneously rotated by a spindle, whereby information is recorded/reproduced/erased by an optical head disposed to confront each of the recording surfaces of the optical discs. Since a large quantity of information items must be stored in the above-described stack type optical disc apparatus, there arises a necessity of further shortening the access time in which information is retrieved by the optical head. Furthermore, the thickness of the optical head must be reduced and the intervals between the discs must also be reduced so as to reduce the dimension of the stack type optical disc in the direction of the spindle.
It is preferable that a separation type optical head disclosed, for example, in Japanese Patent Unexamined Publication No. 61-22448 or Japanese Patent Unexamined Publication No. 61-104338 be employed in order to shorten the access time. The above-disclosed separation type optical head has been arranged in such a manner that the optical head is divided into a fixed portion including a light source portion and a detection portion and a movable portion including a focusing lens and an actuator for controlling the spot position. Furthermore, only the movable portion is moved by a voice coil motor.
The access time, in a case of a structure arranged in such a manner that the separation type optical head is employed, can be shortened in comparison to the conventional integrated type optical head arranged in such a manner that the overall body of the optical head is moved. The reason for this lies in that the weight of the separation type optical head can be reduced since the number of the parts constituting the movable portion can be reduced.
As for the reduction of the thickness of the optical head, a structure capable of reducing the thickness of the optical head has been disclosed in, for example, "Japanese Journal of Applied Physics", Vol. 26, 1987, p.p. 117 to 120 of the supplement. According to this structure, a thin type optical head having a height of 10 mm has been realized wherein an aspheric single lens and a 2-dimensional lens (2-D lens) actuator are employed. Another thin type optical head has been disclosed in "National Technical Report", Vol. 35, No. 2, April 1989, p.p. 67 to 73.
However, all of the above-described separation type optical heads respectively disclosed in Japanese Patent Laid-Open Nos. 61-22448 and 61-104338 and the thin type optical heads respectively disclosed in "Japanese Journal of Applied Physics", Vol. 26, 1987, p.p. 117 to 120 of the supplement, and "National Technical Report", Vol. 35, No. 2, April 1989, p.p. 67 to 73 are respectively arranged to be employed in an optical disc apparatus of a type having one optical disc so as to record/reproduce information. The optical disc medium for use in an optical disc apparatus of the type described above is usually accommodated in a disc cartridge made of, for example, plastic for the purpose of protecting the disc. When the optical disc medium is loaded into the optical disc apparatus together with the disc cartridge, the optical disc medium is rotated at high speed in the disc cartridge. Furthermore, an opening formed in the disc cartridge is partially opened so that laser beams are applied to the surface of the optical disc medium from the optical head. As a result, information recording/reproducing is performed. In the disc cartridge, although air tends to rotate together with the optical disc medium which rotates at high speed, no high speed air flow is generated since the air is surrounded by the stationary disc cartridge. Furthermore, the air flow does not flow from the disc cartridge. Therefore, no special means has been necessary to eliminate the influence of the high speed air flow generated due to the high speed rotation of the optical disc medium.